We breathe without thinking about it. But to control stress, proper breathing must be deep, slow and complete.

You may not realize how much your breathing rhythm changes under stress. Suddenly, your breathing becomes shallow and irregular. You may feel shortness of breath or your heart pounding against your chest.
Guard against shallow breathing by taking in more air through relaxed, controlled inhalations. By filling your lungs with air and releasing the air gradually as you exhale, you improve oxygen flow and increase your lung capacity. Try this exercise:
Stand and place your hand flat on your stomach, just below the navel.
Count to three as you inhale slowly. Your hand should push gently as you breathe in.
Count to three as you exhale slowly. Your hand should cup your stomach as you breathe out.
Repeat four of these slow, complete breaths--two through your nose, then two through your mouth.
Prevent stress by engaging in this breathing exercise three times a day (say, at 9:30 a.m., noon and 4:00 p.m.). Your body will welcome these respites from workday pressures and you'll benefit from the calm that accompanies taking complete breaths.